The other two answers are correct if we consider old grammar rules. Recently, however, the demonstrative pronouns don't have a written accent anymore, unless there is ambiguity. So, "esa es una tarea aburrida" is a perfectly correct sentence if we consider the current rules! :)

Ésa is a demostrative pronoun, it is used instead of a noun; It acts as a subject in a sentence. Ex: Ésa es mi cartera. Esa is a demonstrative adjective and and goes with a noun. Ex: Esa cartera es mia.

When it's an adjective it should be esa, but when it's a pronoun it is ésa. So you're right. Regarding el and él, it seems like you're right as well. You have only written down the beggining of the sentence, but it should be él. It's difficult to know the difference between them, and many native speakers don't know it, so it's common to see these kind of mistakes.